WO2012136848A1 - Bipolar punch-through semiconductor device and method for manufacturing such a semiconductor device - Google Patents

Bipolar punch-through semiconductor device and method for manufacturing such a semiconductor device Download PDF

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Publication number
WO2012136848A1
WO2012136848A1 PCT/EP2012/056456 EP2012056456W WO2012136848A1 WO 2012136848 A1 WO2012136848 A1 WO 2012136848A1 EP 2012056456 W EP2012056456 W EP 2012056456W WO 2012136848 A1 WO2012136848 A1 WO 2012136848A1
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Prior art keywords
layer
doped
doping concentration
conductivity type
region
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PCT/EP2012/056456
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French (fr)
Inventor
Munaf Rahimo
Arnost Kopta
Thomas Clausen
Maxi ANDENNA
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Abb Technology Ag
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Publication date
Application filed by Abb Technology Ag filed Critical Abb Technology Ag
Priority to EP12715066.2A priority Critical patent/EP2695193B1/en
Priority to JP2014503176A priority patent/JP6049690B2/en
Priority to CN201280017459.1A priority patent/CN103597602B/en
Priority to KR1020137029377A priority patent/KR101798273B1/en
Publication of WO2012136848A1 publication Critical patent/WO2012136848A1/en
Priority to US14/046,156 priority patent/US9006041B2/en

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    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
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    • H01L29/66234—Bipolar junction transistors [BJT]
    • H01L29/66325—Bipolar junction transistors [BJT] controlled by field-effect, e.g. insulated gate bipolar transistors [IGBT]
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    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
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    • H01L29/083—Anode or cathode regions of thyristors or gated bipolar-mode devices
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    • H01L29/66234—Bipolar junction transistors [BJT]
    • H01L29/66325—Bipolar junction transistors [BJT] controlled by field-effect, e.g. insulated gate bipolar transistors [IGBT]
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    • H01L29/66234—Bipolar junction transistors [BJT]
    • H01L29/66325—Bipolar junction transistors [BJT] controlled by field-effect, e.g. insulated gate bipolar transistors [IGBT]
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    • H01L29/70—Bipolar devices
    • H01L29/72—Transistor-type devices, i.e. able to continuously respond to applied control signals
    • H01L29/739—Transistor-type devices, i.e. able to continuously respond to applied control signals controlled by field-effect, e.g. bipolar static induction transistors [BSIT]
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    • H01L29/72—Transistor-type devices, i.e. able to continuously respond to applied control signals
    • H01L29/739—Transistor-type devices, i.e. able to continuously respond to applied control signals controlled by field-effect, e.g. bipolar static induction transistors [BSIT]
    • H01L29/7393—Insulated gate bipolar mode transistors, i.e. IGBT; IGT; COMFET
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    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
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    • H01L29/70—Bipolar devices
    • H01L29/72—Transistor-type devices, i.e. able to continuously respond to applied control signals
    • H01L29/739—Transistor-type devices, i.e. able to continuously respond to applied control signals controlled by field-effect, e.g. bipolar static induction transistors [BSIT]
    • H01L29/7393—Insulated gate bipolar mode transistors, i.e. IGBT; IGT; COMFET
    • H01L29/7395—Vertical transistors, e.g. vertical IGBT
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    • H01L29/70—Bipolar devices
    • H01L29/72—Transistor-type devices, i.e. able to continuously respond to applied control signals
    • H01L29/739—Transistor-type devices, i.e. able to continuously respond to applied control signals controlled by field-effect, e.g. bipolar static induction transistors [BSIT]
    • H01L29/7393—Insulated gate bipolar mode transistors, i.e. IGBT; IGT; COMFET
    • H01L29/7395—Vertical transistors, e.g. vertical IGBT
    • H01L29/7396—Vertical transistors, e.g. vertical IGBT with a non planar surface, e.g. with a non planar gate or with a trench or recess or pillar in the surface of the emitter, base or collector region for improving current density or short circuiting the emitter and base regions
    • H01L29/7397—Vertical transistors, e.g. vertical IGBT with a non planar surface, e.g. with a non planar gate or with a trench or recess or pillar in the surface of the emitter, base or collector region for improving current density or short circuiting the emitter and base regions and a gate structure lying on a slanted or vertical surface or formed in a groove, e.g. trench gate IGBT
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Definitions

  • Bipolar punch-through semiconductor device and method for manufacturing such a semiconductor device
  • the invention relates to the field of power electronics and more particularly to a method for manufacturing a bipolar punch-through semiconductor device according to the preamble of claim 1 and to a bipolar punch-through
  • EP 1 017 093 A1 a method for manufacturing an IGBT having an emitter side 14 and a collector side 16 is described.
  • an n doped layer is created by diffusion.
  • a p-base layer 4 n source regions 5 and a gate electrode 6 are then created.
  • an emitter electrode 82 is applied.
  • the thickness of the wafer is reduced on the collector side 16 so that a tail section of the n doped layer remains as a buffer layer 3.
  • a p collector layer 75 and a collector electrode 92 are applied.
  • the n layer from which the buffer layer is made has to be cut within the rising doping profile as shown by the dashed line in Fig. 2, it is difficult to cut the wafer accurately.
  • a cut at a wrong depth can lead to a device, in which the punch-through properties of the IGBT cannot be guaranteed any more, if the doping concentration is too low, or the buffer layer can be made thicker than electrically necessary to assure the punch- through properties.
  • a thicker buffer layer creates higher losses and variations in bipolar gain.
  • the device has non-uniform current.
  • Such a prior art method is used for devices with blocking voltages up to around 2000 V, because such devices are relatively thin. It would be difficult if such devices were manufactured directly on a thin wafer, because working directly on thin wafers requires rather complex processes if the wafer is thin in low voltage IGBTs for forming the frontside layers including the emitter MOS cells and termination and the backside layers including the anode and buffer regions. However, even with the implementation of the described method above, such devices require optimization for improved static and dynamic performance with a number of limiting process options.
  • EP 0 749 166 A1 shows a diode which is created providing a highly doped wafer, and successively epitaxially growing layers each with constant doping
  • the buffer layer comprises thus a first part of constant doping concentration and another part of decreasing doping concentration.
  • a drift layer is also grown also with a plurality of layers of decreasing doping
  • the p doped anode layer is a diffused layer.
  • This object is achieved by a method for manufacturing a bipolar punch-through semiconductor device according to claim 1 and by a bipolar punch-through semiconductor device according to claim 8.
  • a bipolar punch-through semiconductor device with a semiconductor wafer which comprises depending on the semiconductor type at least a two-layer structure with layers of a first and a second conductivity type, which second conductivity type is different from the first conductivity type, wherein one of the layers is a drift layer of the first conductivity type.
  • a wafer with a wafer thickness is provided, which has a first side and a second side, wherein on the first side a high-doped layer of the first conductivity type is arranged, which has a constant high doping
  • a low-doped layer of the first conductivity type is created by epitaxial
  • a diffusion step is performed, by which a diffused inter-space (interface) region is created, which comprises parts of the original high- doped layer and the low-doped layer. These parts are arranged adjacent to each other.
  • the inter-space region has a doping concentration, which is higher than the doping concentration of the low-doped layer and lower than the doping concentration of the high-doped layer, wherein the remaining part of the low-doped layer forms a drift layer.
  • the wafer thickness is reduced on the second side within the high-doped layer so that a buffer layer is created, which comprises the inter-space region and the remaining part of the high-doped layer, which forms a high-doped region.
  • the doping profile of the buffer layer decreases steadily from the uniform (i.e. constant) doping concentration of the high- doped region to the uniform (i.e. constant) doping concentration of the drift layer.
  • the new buffer design provides a similar final thickness to the prior art soft punch- through designs while eliminating many process issues related to prior art buffer formation processes. For example, a much better control of the final thickness of depth, in which the high-doped layer is thinned in order to create the buffer layer is achieved, because the thinning is done in a non profiled part of the high-doped layer, i.e. in a part of constant doping concentration. That means that no grinding or etching is performed within a rising doping concentration gradient, which could otherwise lead to variations in the bipolar gain and non-uniform current flow under different conditions.
  • the drift layer is of high quality due to the epitaxial process while the buffer layer can utilize less stringent specifications with a lower cost impact if a homogeneously doped wafer is used as a starting material.
  • the process and design can be easily adapted to larger wafer diameter processing.
  • deep diffused buffer layers are difficult to create for larger wafers, because buffer formation during the process requires thin wafer handling at very early stages, hence the need for wafer carrier process solutions.
  • better handling is possible since the process requires only thin wafer handling at the backend stage compared to other buffer designs and processes and even controllable processes even for large wafers.
  • the inventive devices provide in terms of device performance better design control and processes with lower leakage currents, improved short circuit capability and softer turn-off behavior.
  • FIG 1 shows a cross sectional view on a prior art IGBT with planar gate
  • FIG 2 shows a doping profile of the prior art IGBT according to FIG 1 ;
  • FIG 3 shows a cross sectional view on an inventive IGBT with planar gate electrode;
  • FIG 4 shows a doping profile of the inventive IGBT according to FIG 3;
  • FIG 5 to 9 show manufacturing steps for manufacturing an inventive
  • FIG 10 show a manufacturing step for manufacturing an inventive diode
  • FIG 11 shows a cross sectional view on an inventive diode
  • FIG 12 shows a cross sectional view on an inventive trench IGBT.
  • a bipolar punch-through semiconductor device as shown in the Fig.s 3, 11 and 12 comprises a semiconductor wafer 1 , also called semiconductor substrate, with a first main side 14 and a second main side 16, which is arranged opposite of the first main side 14.
  • a first electrical contact 8 is arranged on the first main side 14, and a second electrical contact 9 is arranged on the second main side 16.
  • the device has at least a two-layer structure with layers of a first and a second conductivity type, which second conductivity type is different from the first conductivity type.
  • One of the layers is a low-doped drift layer 2 of a first conductivity type, i.e. of n type.
  • the inventive device as shown in Fig. 3 is an insulated gate bipolar transistor (IGBT) 10, in which the first electrical contact is formed as an emitter electrode 82 and the second electrical contact 9 is formed as a collector electrode 92.
  • IGBT insulated gate bipolar transistor
  • a non-punch through power semiconductor device is a device, in which the drift layer of the first conductivity type is in contact to the collector layer without having a highly doped layer of the first conductivity type (called buffer layer) in between.
  • the electric field in blocking condition for a non punch-through device is triangular and stops within the drift layer. The space charge region does not reach the collector layer.
  • a device comprising such a buffer layer (which buffer layer has higher doping concentration than the drift layer) is called a punch-through device. At higher blocking voltages the electric field at the border between the drift and buffer layer will not have reached zero. Along a short distance in the buffer layer it is then steeply decreased to zero due to the high doping concentration.
  • a p type layer in form of a base layer 4 is arranged on the first main side 14 (emitter side). At least one n type source region 5 is arranged on the emitter side 14 and is surrounded by the base layer 4. The at least one source region 5 has a higher doping than the drift layer 2.
  • a first electrically insulating layer 62 is arranged on the emitter side 14 on top of the drift layer 2, the base layer 4 and the source region 5. It at least partially covers the source region 5, the base layer 4 and the drift layer 2.
  • An electrically conductive gate electrode 6 is arranged on the emitter side 14 electrically insulated from the at least one base layer 4, the source regions 5 and the drift layer 2 by the electrically insulating layer 62, which is typically made of silicon dioxide.
  • the gate electrode 6 is embedded in the electrically insulating layer 62 and covered by another second insulating layer 64, preferably of the same material as the first insulating layer 62.
  • the choice of the doping concentration and thicknesses of the drift layer 2 depends on the blocking capability requirements.
  • the low-doped drift layer 2 is the main region for supporting the blocking voltage on the main PN junction side
  • Typical thicknesses of a drift layer for a 600 V device is 30 to 70 ⁇ , 80 to 120 ⁇ for a 1200 V device and 150 to 190 ⁇ for a 1700 V device.
  • the doping concentration for a lower voltage device is typically higher than for a higher voltage device, e.g. around 1.5 * 10 14 cm “3 for a 600 V device down to 5 * 10 13 cm "3 for a 1700 V device.
  • the concrete values for a device may vary depending on its application.
  • the first electrically insulating region 62 is arranged on top of the emitter side. In between the first and second electrically insulating layers 62, 64, the gate electrode 6 is embedded, typically it is completely embedded.
  • the gate electrode 6 is typically made of a heavily doped polysilicon or a metal like aluminum.
  • the at least one source region 5, the gate electrode 6 and the electrically insulating layers 62, 64 are formed in such a way that an opening is created above the base layer 4. The opening is surrounded by the at least one source region 5, the gate electrode 6 and the electrically insulating layers 62, 64.
  • the first electrical contact 8 is arranged on the first main side 14 covering the opening so that it is in direct electrical contact to the base layer 4 and the source regions 5.
  • This first electrical contact 8 typically also covers the electrically insulating layers 62, 64, but is separated and thus electrically insulated from the gate electrode 6 by the second electrically insulating layer 64.
  • the inventive IGBT may comprise a gate electrode formed as trench gate electrode 6' as shown in FIG 12.
  • the trench gate electrode 6' is arranged in the same plane as the base layer 4 and adjacent to the source regions 5, separated from each other by a first insulating layer 62, which also separates the gate electrode 6 from the drift layer 2.
  • a second insulating layer 64 is arranged on top of the gate electrode formed as a trench gate electrode 9', thus insulating the trench gate electrode 6' from the first electrical contact 8 (FIG 12).
  • the buffer layer 3 comprises towards the second main side 16 a high-doped region 36, which is constantly high-doped, and between the high-doped region 36 and the drift layer 2 an inter-space region 32, which is a diffused region, which means that it has a doping concentration, which decreases steadily from the doping concentration of the uniformly high-doped region to the uniform low doping concentration of the drift layer.
  • the high-doped region 36 shall be a region with higher doping concentration than the low-doped drift layer 2.
  • the doping concentration decreases typically by a Gaussian function. However, if by diffusion another continuously decreasing profile of the doping concentration is achieved, this shall be also covered by the invention.
  • a collector layer 75 is arranged on the second main side 16 between the buffer layer 3 and the collector electrode 92.
  • the inventive bipolar punch-through semiconductor devices can also be reverse conducting IGBTs with alternating p doped collector layer 75 and n+ doped additional layers in a plane parallel to the second main side 16.
  • FIG 1 1 an inventive bipolar punch-through semiconductor device in form of a bipolar diode 12 is shown.
  • the diode 12 comprises a drift layer 2 of a first conductivity type, i.e. of n type, with a first main side 14 and a second main side 16 opposite the first main side 14.
  • a p doped layer in form of an anode layer 7 is arranged on the first main side 14.
  • a first electrical contact 8 as an anode electrode 84, typically in form of a metal layer is arranged on top of the anode layer 7, i.e. on that side of the layer 7, which lies opposite the drift layer 2.
  • an inventive (n) doped buffer layer 3 is arranged.
  • This buffer layer 3 has a higher doping concentration than the drift layer 2.
  • the buffer layer 3 comprises a constantly high-doped layer 36, which is arranged towards the anode electrode 94 and an inter-space region 32, which is arranged between the high-doped region 36 and the drift layer 2.
  • the inter-space region 32 is designed as disclosed above for the inventive IGBT 10.
  • Any inventive bipolar punch-through semiconductor device can for example be used in a converter.
  • a wafer with a wafer thickness is provided, which has a first side 15 and a second side 17, wherein on the first side 15 a high-doped n type layer 34 is arranged, which has a constant high doping concentration (FIG 5).
  • the high-doped layer 34 has a doping concentration of 5 * 10 14 to 5 * 10 16 cm "3 .
  • a low-doped n type layer 22 is created by epitaxial growth on the first side 15 (FIG 6) on top of the high-doped layer 34.
  • a layer is achieved with constant doping concentration. That means that the layer shows a uniform doping concentration.
  • the low-doped layer 22 is created with a doping concentration of 3 * 10 13 cm “3 to 2 * 10 14 cm “3 .
  • a diffusion step is performed, by which a diffused inter-space region 32 is created, which comprises parts of the high-doped layer 34 and the low-doped layer 22, which parts are arranged adjacent to each other (FIG 8).
  • the inter-space region 32 has a doping concentration, which is higher than the doping concentration of the low-doped layer and lower than the doping concentration of the high-doped layer, wherein the remaining part of the low-doped layer forms a drift layer 2.
  • Fig. 8 the original border 35 between the high-doped and low-doped layer 34, 22 is shown as a dotted line.
  • At least one layer of the second conductivity type is created on the first side 15 on top of the drift layer 2 (FIG 9, which shows the manufacturing method for a diode).
  • the p doped layer can also be diffused into the drift layer 2 such that the p doped layer is arranged on the first main side 15 with the drift layer 2 below the p doped layer.
  • the thickness of the wafer is reduced on the second side 17 within the high-doped layer 34 so that a buffer layer 3 is created, which comprises the inter-space region 32 and at least a remaining part of the high-doped layer, which forms a high-doped region 36 (FIG 10, in which the method is exemplarily shown for the creation of a diode).
  • a buffer layer 3 is created, which comprises the inter-space region 32 and at least a remaining part of the high-doped layer, which forms a high-doped region 36 (FIG 10, in which the method is exemplarily shown for the creation of a diode).
  • Any appropriate method well-known to the experts can be used for the reduction of the thickness like grinding or etching.
  • the thickness is reduced by removing a part of the wafer over the whole plane of the wafer on the second side and parallel to the second side.
  • the doping profile of the buffer layer decreases steadily from the doping concentration of the high- doped region to the doping concentration of the drift
  • the wafer, which is provided for step (a) can be a complete n type wafer, which is constantly high-doped.
  • the wafer thickness shall be the thickness of the wafer between the first and second side 15, 17 in step (a).
  • the whole wafer 1 may comprise on the second side 17 a mounting layer 18, which is completely removed in step (e) and a high doped layer 34 on the first side 15(shown in FIG 7).
  • the mounting layer 18 can be a low cost material, typically of n type or p type. Examplarily, it is a p-type high-doped layer.
  • the high-doped layer 34 is typically created by epitaxial growth, because the high- doped layer 34 (as well as the low doped layer 22) have to be of constant doping concentration.
  • the further manufacturing steps are the same as disclosed above for the uniform n-type wafer.
  • the diffusion is exemplarily performed at a temperature of at least 1200 °C and during a time period of at least 180 min.
  • step (c) by the diffusion particles from the high-doped layer 34 diffuse into the low-doped layer 22 so that an inter-space region 32 is created, which comprises such part from the high-doped layer 34, from which particles have been diffused and such part of the low-doped layer 22, into which the particles from the high-doped layer 34 have been diffused to.
  • the drift layer 2 is such part of the low-doped layer 22 with unamended low doping concentration, whereas the buffer layer 3 comprises such regions towards the second side 17, which are n type and which have higher doping concentration than the drift layer 2. That is the inter-space region 32 and such part of the high- doped layer 34, which after thinning and diffusion remains with the high doping concentration. This part forms the high-doped region 36.
  • Fig. 4 shows the doping concentration within the wafer for a uniform n type wafer in different manufacturing steps
  • the dashed line shows the doping concentration of the high-doped layer 34 and low doped layer 22 after implantation (step (b)).
  • the continuous line shows the wafer after diffusion (step (c)) and the dotted line shows the wafer after the p type layer is created on the first main side (step (d)).
  • step (d) in case of a diode 12, an anode layer 7 is created.
  • the first electrical contact 8 formed as an anode electrode 84 may be created at this step, typically be deposition of metal on the first and second side 15, 17. Alternatively, the anode electrode 84 may be created together with the cathode electrode 94 after the thinning of step (e).
  • step (d) in case of an IGBT 10, the p base layer 4 and the source region 5 are created on the first side 15, Afterwards the planar gate electrode 6 or trench gate electrode 6' is created on the first side together with its insulation layers 62, 64.
  • the first electrical contact 8 formed as emitter electrode 82 may be made on the first side 15 at this step (d). Alternatively, the emitter electrode 82 may be created together with the collector electrode 92 after the thinning of step (e).
  • the p collector layer 75 is created and on the second side 17 after the thinning of step (e), but of course before the creation of the collector electrode 92.
  • a doped collector layer alternating with an n+ doped additional layer is created in a plane parallel to the second side 17 after thinning (step (e)) by using masking techniques well-known to experts.
  • a typical thickness of the buffer layer (3) is (20 - 40) ⁇ .
  • the buffer layer 3 always comprises towards the second main side 16 a high-doped region 36, i.e. a section, which has constant doping concentration.
  • the inter-space region 32 between the high-doped region 36 and the drift layer 2 is in any case thinner than the buffer layer 3 due to the presence of the high-doped region 36.
  • the inter-space region 32 has a thickness of (10 - 30) ⁇ .
  • the buffer layer 3 After the buffer layer 3 has been created other layers may be created in or on the wafer 1 on the second side 17 and after thinning.
  • the p doped collector layer 75 and the collector electrode 92 are now created.
  • the conductivity types are switched, i.e. all layers of the first conductivity type are p type (e.g. the drift layer 2, the source region 5) and all layers of the second conductivity type are n type (e.g. base layer 4, the collector layer 75).

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Abstract

A method for manufacturing a bipolar punch-through semiconductor device is provided, wherein the following steps are performed (a) a wafer (1) having a first and a second side (15, 17) is provided, wherein on the first side (17) a high-doped layer (34) of the first conductivity type having constant high doping concentration is arranged, (b) a low-doped layer (22) of the first conductivity type is epitaxially grown on the first side (15), (c) afterwards a diffusion step is performed, by which a diffused inter-space region (32) is created at the inter-space of the layers (34, 22), (d) afterwards at least one layer of the second conductivity type is created on the first side (15), (e) afterwards the wafer thickness is reduced within the high-doped layer (34) is on the second side (17) so that a buffer layer (3) is created, which comprises the inter-space region (32) and the remaining part of the high- doped layer, wherein the doping profile of the buffer layer (3) decreases steadily from the doping concentration of the high-doped region (36) to the doping concentration of the drift layer (2).

Description

Bipolar punch-through semiconductor device and method for manufacturing such a semiconductor device
Description
Technical Field
The invention relates to the field of power electronics and more particularly to a method for manufacturing a bipolar punch-through semiconductor device according to the preamble of claim 1 and to a bipolar punch-through
semiconductor device according to the preamble of claim 6.
Background Art
In EP 1 017 093 A1 a method for manufacturing an IGBT having an emitter side 14 and a collector side 16 is described. On the collector side 16 of a wafer an n doped layer is created by diffusion. On the emitter side 14, a p-base layer 4, n source regions 5 and a gate electrode 6 are then created. Afterwards an emitter electrode 82 is applied. The thickness of the wafer is reduced on the collector side 16 so that a tail section of the n doped layer remains as a buffer layer 3. Finally a p collector layer 75 and a collector electrode 92 are applied. By such a method an IGBT is created which has a lowly doped buffer layer 3. Such devices are, therefore, called soft punch-through devices. However, as the n layer from which the buffer layer is made, has to be cut within the rising doping profile as shown by the dashed line in Fig. 2, it is difficult to cut the wafer accurately. A cut at a wrong depth can lead to a device, in which the punch-through properties of the IGBT cannot be guaranteed any more, if the doping concentration is too low, or the buffer layer can be made thicker than electrically necessary to assure the punch- through properties. However, a thicker buffer layer creates higher losses and variations in bipolar gain. Furthermore, the device has non-uniform current.
Such a prior art method is used for devices with blocking voltages up to around 2000 V, because such devices are relatively thin. It would be difficult if such devices were manufactured directly on a thin wafer, because working directly on thin wafers requires rather complex processes if the wafer is thin in low voltage IGBTs for forming the frontside layers including the emitter MOS cells and termination and the backside layers including the anode and buffer regions. However, even with the implementation of the described method above, such devices require optimization for improved static and dynamic performance with a number of limiting process options.
Similar challenges are met when designing fast recovery diodes based on thin wafer processing. In addition, the larger the wafer diameter, the more the difficulties encountered for thin wafer processing. Finally, the quality and availability of silicon substrate material is also an issue for thin wafer technologies utilizing for example deep diffused methods especially for larger wafer diameters above 200 mm.
EP 0 749 166 A1 shows a diode which is created providing a highly doped wafer, and successively epitaxially growing layers each with constant doping
concentration, but in the series with decreasing doping concentration to form intermediate buffer layer. The buffer layer comprises thus a first part of constant doping concentration and another part of decreasing doping concentration. A drift layer is also grown also with a plurality of layers of decreasing doping
concentration. The p doped anode layer is a diffused layer. As a diode is manufactured in EP 0 749 166 A1 , no sophisticated structure is needed on the cathode (emitter) side like in a design of an IGBT.
The method applied in EP 0 749 166 A1 is complicated, time consuming and expensive due to the necessity of growing a plurality of epitaxial layers and even more complicated due to the growing the subsequent epitaxial layers with decreasing doping concentration and ending in a device with a thick buffer layer due to a thick highly doped wafer being used as a basis for the manufacturing. Disclosure of Invention
It is an object of the invention to provide a method for manufacturing a bipolar punch-through semiconductor device, which is applicable for low-voltage devices and by which method a better controllability of the manufacturing method itself and consequently of the electrical properties of the device are achievable.
This object is achieved by a method for manufacturing a bipolar punch-through semiconductor device according to claim 1 and by a bipolar punch-through semiconductor device according to claim 8.
With the inventive method a bipolar punch-through semiconductor device with a semiconductor wafer is manufactured, which comprises depending on the semiconductor type at least a two-layer structure with layers of a first and a second conductivity type, which second conductivity type is different from the first conductivity type, wherein one of the layers is a drift layer of the first conductivity type.
In the inventive method the following manufacturing steps are performed:
(a) A wafer with a wafer thickness is provided, which has a first side and a second side, wherein on the first side a high-doped layer of the first conductivity type is arranged, which has a constant high doping
concentration.
(b) A low-doped layer of the first conductivity type is created by epitaxial
growth on the first side.
(c) Afterwards a diffusion step is performed, by which a diffused inter-space (interface) region is created, which comprises parts of the original high- doped layer and the low-doped layer. These parts are arranged adjacent to each other. The inter-space region has a doping concentration, which is higher than the doping concentration of the low-doped layer and lower than the doping concentration of the high-doped layer, wherein the remaining part of the low-doped layer forms a drift layer.
(d) Afterwards at least one layer of the second conductivity type is created on the first side.
(e) Afterwards the wafer thickness is reduced on the second side within the high-doped layer so that a buffer layer is created, which comprises the inter-space region and the remaining part of the high-doped layer, which forms a high-doped region. The doping profile of the buffer layer decreases steadily from the uniform (i.e. constant) doping concentration of the high- doped region to the uniform (i.e. constant) doping concentration of the drift layer.
The new buffer design provides a similar final thickness to the prior art soft punch- through designs while eliminating many process issues related to prior art buffer formation processes. For example, a much better control of the final thickness of depth, in which the high-doped layer is thinned in order to create the buffer layer is achieved, because the thinning is done in a non profiled part of the high-doped layer, i.e. in a part of constant doping concentration. That means that no grinding or etching is performed within a rising doping concentration gradient, which could otherwise lead to variations in the bipolar gain and non-uniform current flow under different conditions.
Furthermore, the drift layer is of high quality due to the epitaxial process while the buffer layer can utilize less stringent specifications with a lower cost impact if a homogeneously doped wafer is used as a starting material.
The process and design can be easily adapted to larger wafer diameter processing. In prior art methods, deep diffused buffer layers are difficult to create for larger wafers, because buffer formation during the process requires thin wafer handling at very early stages, hence the need for wafer carrier process solutions. According to this invention, better handling is possible since the process requires only thin wafer handling at the backend stage compared to other buffer designs and processes and even controllable processes even for large wafers.
The inventive devices provide in terms of device performance better design control and processes with lower leakage currents, improved short circuit capability and softer turn-off behavior.
Further preferred embodiments of the inventive subject matter are disclosed in the dependent claims.
Brief Description of Drawings
The subject matter of the invention will be explained in more detail in the following text with reference to the attached drawings, in which:
FIG 1 shows a cross sectional view on a prior art IGBT with planar gate
electrode;
FIG 2 shows a doping profile of the prior art IGBT according to FIG 1 ; FIG 3 shows a cross sectional view on an inventive IGBT with planar gate electrode;
FIG 4 shows a doping profile of the inventive IGBT according to FIG 3;
FIG 5 to 9 show manufacturing steps for manufacturing an inventive
semiconductor device;
FIG 10 show a manufacturing step for manufacturing an inventive diode;
FIG 11 shows a cross sectional view on an inventive diode; and
FIG 12 shows a cross sectional view on an inventive trench IGBT.
The reference symbols used in the figures and their meaning are summarized in the list of reference symbols. Generally, alike or alike-functioning parts are given the same reference symbols. The described embodiments are meant as examples and shall not confine the invention.
Modes for Carrying out the Invention
A bipolar punch-through semiconductor device according to the invention as shown in the Fig.s 3, 11 and 12 comprises a semiconductor wafer 1 , also called semiconductor substrate, with a first main side 14 and a second main side 16, which is arranged opposite of the first main side 14. A first electrical contact 8 is arranged on the first main side 14, and a second electrical contact 9 is arranged on the second main side 16. The device has at least a two-layer structure with layers of a first and a second conductivity type, which second conductivity type is different from the first conductivity type. One of the layers is a low-doped drift layer 2 of a first conductivity type, i.e. of n type.
The inventive device as shown in Fig. 3 is an insulated gate bipolar transistor (IGBT) 10, in which the first electrical contact is formed as an emitter electrode 82 and the second electrical contact 9 is formed as a collector electrode 92.
As well-known to the experts, a non-punch through power semiconductor device is a device, in which the drift layer of the first conductivity type is in contact to the collector layer without having a highly doped layer of the first conductivity type (called buffer layer) in between. The electric field in blocking condition for a non punch-through device is triangular and stops within the drift layer. The space charge region does not reach the collector layer. A device comprising such a buffer layer (which buffer layer has higher doping concentration than the drift layer) is called a punch-through device. At higher blocking voltages the electric field at the border between the drift and buffer layer will not have reached zero. Along a short distance in the buffer layer it is then steeply decreased to zero due to the high doping concentration.
A p type layer in form of a base layer 4 is arranged on the first main side 14 (emitter side). At least one n type source region 5 is arranged on the emitter side 14 and is surrounded by the base layer 4. The at least one source region 5 has a higher doping than the drift layer 2. A first electrically insulating layer 62 is arranged on the emitter side 14 on top of the drift layer 2, the base layer 4 and the source region 5. It at least partially covers the source region 5, the base layer 4 and the drift layer 2. An electrically conductive gate electrode 6 is arranged on the emitter side 14 electrically insulated from the at least one base layer 4, the source regions 5 and the drift layer 2 by the electrically insulating layer 62, which is typically made of silicon dioxide. Preferably, the gate electrode 6 is embedded in the electrically insulating layer 62 and covered by another second insulating layer 64, preferably of the same material as the first insulating layer 62.
The choice of the doping concentration and thicknesses of the drift layer 2 depends on the blocking capability requirements. The low-doped drift layer 2 is the main region for supporting the blocking voltage on the main PN junction side
(emitter for IGBT, anode for diode) while the higher doped buffer layer is near the second main side (collector side for IGBT or cathode side in case of a diode). Typical thicknesses of a drift layer for a 600 V device is 30 to 70 μηι, 80 to 120 μηι for a 1200 V device and 150 to 190 μηι for a 1700 V device. The doping concentration for a lower voltage device is typically higher than for a higher voltage device, e.g. around 1.5 * 1014 cm"3 for a 600 V device down to 5 * 1013 cm"3 for a 1700 V device. However, the concrete values for a device may vary depending on its application.
For an IGBT with a gate electrode formed as a planar gate electrode 9 as shown in FIG 3 the first electrically insulating region 62 is arranged on top of the emitter side. In between the first and second electrically insulating layers 62, 64, the gate electrode 6 is embedded, typically it is completely embedded. The gate electrode 6 is typically made of a heavily doped polysilicon or a metal like aluminum. The at least one source region 5, the gate electrode 6 and the electrically insulating layers 62, 64 are formed in such a way that an opening is created above the base layer 4. The opening is surrounded by the at least one source region 5, the gate electrode 6 and the electrically insulating layers 62, 64.
The first electrical contact 8 is arranged on the first main side 14 covering the opening so that it is in direct electrical contact to the base layer 4 and the source regions 5. This first electrical contact 8 typically also covers the electrically insulating layers 62, 64, but is separated and thus electrically insulated from the gate electrode 6 by the second electrically insulating layer 64.
Alternatively to the inventive IGBT with a planar gate electrode 6, the inventive IGBT may comprise a gate electrode formed as trench gate electrode 6' as shown in FIG 12. The trench gate electrode 6' is arranged in the same plane as the base layer 4 and adjacent to the source regions 5, separated from each other by a first insulating layer 62, which also separates the gate electrode 6 from the drift layer 2. A second insulating layer 64 is arranged on top of the gate electrode formed as a trench gate electrode 9', thus insulating the trench gate electrode 6' from the first electrical contact 8 (FIG 12).
A buffer layer 3, which has a higher doping concentration than the drift layer 2, is arranged on the drift layer 2 towards the second main side 16. The buffer layer 3 comprises towards the second main side 16 a high-doped region 36, which is constantly high-doped, and between the high-doped region 36 and the drift layer 2 an inter-space region 32, which is a diffused region, which means that it has a doping concentration, which decreases steadily from the doping concentration of the uniformly high-doped region to the uniform low doping concentration of the drift layer. The high-doped region 36 shall be a region with higher doping concentration than the low-doped drift layer 2.
In the diffused inter-space region 32 the doping concentration decreases typically by a Gaussian function. However, if by diffusion another continuously decreasing profile of the doping concentration is achieved, this shall be also covered by the invention.
A collector layer 75 is arranged on the second main side 16 between the buffer layer 3 and the collector electrode 92. The inventive bipolar punch-through semiconductor devices can also be reverse conducting IGBTs with alternating p doped collector layer 75 and n+ doped additional layers in a plane parallel to the second main side 16.
In FIG 1 1 an inventive bipolar punch-through semiconductor device in form of a bipolar diode 12 is shown. The diode 12 comprises a drift layer 2 of a first conductivity type, i.e. of n type, with a first main side 14 and a second main side 16 opposite the first main side 14. A p doped layer in form of an anode layer 7 is arranged on the first main side 14. A first electrical contact 8 as an anode electrode 84, typically in form of a metal layer is arranged on top of the anode layer 7, i.e. on that side of the layer 7, which lies opposite the drift layer 2.
Towards the second main side 16, an inventive (n) doped buffer layer 3 is arranged. This buffer layer 3 has a higher doping concentration than the drift layer 2. A second electrical contact 9 as a cathode electrode 94, typically in form of a metal layer, is arranged on top of buffer layer 3, i.e. on that side of the buffer layer 3, which lies opposite the drift layer 2. Again, the buffer layer 3 comprises a constantly high-doped layer 36, which is arranged towards the anode electrode 94 and an inter-space region 32, which is arranged between the high-doped region 36 and the drift layer 2. The inter-space region 32 is designed as disclosed above for the inventive IGBT 10.
Any inventive bipolar punch-through semiconductor device can for example be used in a converter.
For manufacturing an inventive bipolar punch-through semiconductor device the following steps are performed:
(a) A wafer with a wafer thickness is provided, which has a first side 15 and a second side 17, wherein on the first side 15 a high-doped n type layer 34 is arranged, which has a constant high doping concentration (FIG 5).
Typically, the high-doped layer 34 has a doping concentration of 5 * 1014 to 5 * 1016 cm"3.
(b) A low-doped n type layer 22 is created by epitaxial growth on the first side 15 (FIG 6) on top of the high-doped layer 34. By epitaxial growth, a layer is achieved with constant doping concentration. That means that the layer shows a uniform doping concentration. Exemplarily, the low-doped layer 22 is created with a doping concentration of 3 * 1013 cm"3 to 2 * 1014 cm"3. (c) Afterwards a diffusion step is performed, by which a diffused inter-space region 32 is created, which comprises parts of the high-doped layer 34 and the low-doped layer 22, which parts are arranged adjacent to each other (FIG 8). The inter-space region 32 has a doping concentration, which is higher than the doping concentration of the low-doped layer and lower than the doping concentration of the high-doped layer, wherein the remaining part of the low-doped layer forms a drift layer 2. In Fig. 8 the original border 35 between the high-doped and low-doped layer 34, 22 is shown as a dotted line.
(d) Afterwards at least one layer of the second conductivity type is created on the first side 15 on top of the drift layer 2 (FIG 9, which shows the manufacturing method for a diode). Of course, the p doped layer can also be diffused into the drift layer 2 such that the p doped layer is arranged on the first main side 15 with the drift layer 2 below the p doped layer.
(e) Afterwards the thickness of the wafer is reduced on the second side 17 within the high-doped layer 34 so that a buffer layer 3 is created, which comprises the inter-space region 32 and at least a remaining part of the high-doped layer, which forms a high-doped region 36 (FIG 10, in which the method is exemplarily shown for the creation of a diode). Any appropriate method well-known to the experts can be used for the reduction of the thickness like grinding or etching. The thickness is reduced by removing a part of the wafer over the whole plane of the wafer on the second side and parallel to the second side. The doping profile of the buffer layer decreases steadily from the doping concentration of the high- doped region to the doping concentration of the drift layer.
The wafer, which is provided for step (a) can be a complete n type wafer, which is constantly high-doped. The wafer thickness shall be the thickness of the wafer between the first and second side 15, 17 in step (a). Alternatively, the whole wafer 1 may comprise on the second side 17 a mounting layer 18, which is completely removed in step (e) and a high doped layer 34 on the first side 15(shown in FIG 7). The mounting layer 18 can be a low cost material, typically of n type or p type. Examplarily, it is a p-type high-doped layer. As it is completely removed, there are no high requirements to the mounting layer 18 besides that it must be possible to create a high-doped layer 34 and low doped layer 22 on the mounting layer. The high-doped layer 34 is typically created by epitaxial growth, because the high- doped layer 34 (as well as the low doped layer 22) have to be of constant doping concentration. The further manufacturing steps are the same as disclosed above for the uniform n-type wafer.
The diffusion is exemplarily performed at a temperature of at least 1200 °C and during a time period of at least 180 min. In step (c) by the diffusion particles from the high-doped layer 34 diffuse into the low-doped layer 22 so that an inter-space region 32 is created, which comprises such part from the high-doped layer 34, from which particles have been diffused and such part of the low-doped layer 22, into which the particles from the high-doped layer 34 have been diffused to. The drift layer 2 is such part of the low-doped layer 22 with unamended low doping concentration, whereas the buffer layer 3 comprises such regions towards the second side 17, which are n type and which have higher doping concentration than the drift layer 2. That is the inter-space region 32 and such part of the high- doped layer 34, which after thinning and diffusion remains with the high doping concentration. This part forms the high-doped region 36.
Fig. 4 shows the doping concentration within the wafer for a uniform n type wafer in different manufacturing steps, the dashed line shows the doping concentration of the high-doped layer 34 and low doped layer 22 after implantation (step (b)). The continuous line shows the wafer after diffusion (step (c)) and the dotted line shows the wafer after the p type layer is created on the first main side (step (d)). In step (d), in case of a diode 12, an anode layer 7 is created. The first electrical contact 8 formed as an anode electrode 84 may be created at this step, typically be deposition of metal on the first and second side 15, 17. Alternatively, the anode electrode 84 may be created together with the cathode electrode 94 after the thinning of step (e).
In step (d), in case of an IGBT 10, the p base layer 4 and the source region 5 are created on the first side 15, Afterwards the planar gate electrode 6 or trench gate electrode 6' is created on the first side together with its insulation layers 62, 64. The first electrical contact 8 formed as emitter electrode 82 may be made on the first side 15 at this step (d). Alternatively, the emitter electrode 82 may be created together with the collector electrode 92 after the thinning of step (e). The p collector layer 75 is created and on the second side 17 after the thinning of step (e), but of course before the creation of the collector electrode 92. In case of a reverse conducting IGBTs a doped collector layer alternating with an n+ doped additional layer is created in a plane parallel to the second side 17 after thinning (step (e)) by using masking techniques well-known to experts.
A typical thickness of the buffer layer (3) is (20 - 40) μηι. The buffer layer 3 always comprises towards the second main side 16 a high-doped region 36, i.e. a section, which has constant doping concentration. The inter-space region 32 between the high-doped region 36 and the drift layer 2 is in any case thinner than the buffer layer 3 due to the presence of the high-doped region 36. Typically the inter-space region 32 has a thickness of (10 - 30) μηι.
After the buffer layer 3 has been created other layers may be created in or on the wafer 1 on the second side 17 and after thinning. For creating an IGBT, for example the p doped collector layer 75 and the collector electrode 92 are now created. Of course, it shall not be excluded from the invention to create layers on the first side 15 after thinning.
These examples shall not limit the scope of the invention. The above mentioned designs and arrangements are just examples for any kinds of possible designs and arrangements for the base layer(s) and well (zones).
In another embodiment, the conductivity types are switched, i.e. all layers of the first conductivity type are p type (e.g. the drift layer 2, the source region 5) and all layers of the second conductivity type are n type (e.g. base layer 4, the collector layer 75).
It should be noted that the term "comprising" does not exclude other elements or steps and that the indefinite article "a" or "an" does not exclude the plural. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.
It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein. Reference List
1 wafer
10 IGBT
12 diode
14 first main side
15 first side
16 second side
17 second main side
18 mounting layer
2 drift layer
21 thickness of drift layer
22 low-doped layer
24 low-doped region
25 thickness of low-doped region
3 buffer layer
31 thickness of buffer layer
32 Inter-space region
33 thickness of inter-space region
34 high-doped layer
35 original border between low-doped and high-doped layer
36 high-doped region
37 thickness of high-doped region
4 base layer
5 source region
6 gate electrode
62 first insulating layer
64 second insulating layer
7 anode layer
75 collector layer
8 first electrical contact
82 emitter electrode
84 cathode electrode
9 second electrical contact
92 collector electrode
94 anode electrode

Claims

C L A I M S
1. Method for manufacturing a bipolar punch-through semiconductor device
having at least a two-layer structure with layers of a first and a second conductivity type, which second conductivity type is different from the first conductivity type, wherein for the creation of the semiconductor device the following steps are performed:
(a) a wafer (1) is provided, which has a first side (15) and a second side (17) and a wafer thickness, wherein on the first side (17) a high-doped layer (34) of the first conductivity type is arranged, which has a constant high doping concentration,
(b) a low-doped layer (22) of the first conductivity type is created by epitaxial growth on the first side (15),
(c) afterwards a diffusion step is performed, by which a diffused inter-space region (32) is created, which comprises parts of the high-doped layer (34) and the low-doped layer (22), which parts are arranged adjacent to each other, which inter-space region (32) has a doping concentration, which is higher than the doping concentration of the low-doped layer and lower than the doping concentration of the high-doped layer, wherein the remaining part of the low-doped layer forms a drift layer (2),
(d) afterwards at least one layer of the second conductivity type is created on the first side (15),
(e) afterwards the wafer thickness is reduced on the second side (17) within the high-doped layer (34), so that a buffer layer (3) is created, which comprises the inter-space region (32) and the remaining part of the high- doped layer, which forms a high-doped region (36), wherein the doping profile of the buffer layer (3) decreases steadily from the doping concentration of the high-doped region (36) to the doping concentration of the drift layer (2).
2. Method according to claim 1 , characterized in that the wafer (1) is completely made of the first conductivity type and constantly doped with a high doping concentration.
3. Method according to claim 1 , characterized in that the wafer (1) in step (a) comprises on the second side (17) a mounting layer (18) which is completely removed in step (e). Method according to any of the claims 1 to 3, characterized in that at least one of
- the thickness of the wafer is reduced in step (e) such that the buffer layer (3) has a thickness (31) of (20 - 40) μητι,
- the inter-space region (32) is created in step (c) such that the interspace region (32) region has a thickness (33) of (10 - 30) μηι, and
- the high-doped layer (34) has a doping concentration of 5 * 1014 to 5 * 1016 cm"3.
Method according to any of the claims 1 to 4, characterized in that the diffusion step is performed at least one of
- at a temperature of at least 1200 °C and
- during a time period of at least 180 min.
Method according to any of the claims 1 to 5, characterized in that the low- doped layer (22) is created with a doping concentration of 3 * 1013 cm"3 to 2 * 1014 cm"3.
Method according to any of the claims 1 to 6, characterized in that the device is an insulated gate bipolar transistor (10) or in that the device is a diode (12). Bipolar punch-through semiconductor device in form of an insulated gate bipolar transistor having a four-layer structure with layers of a first and a second conductivity type, which second conductivity type is different from the first conductivity type, between a first main side (14) and a second main side (16), which comprises
- a drift layer (2) of the first conductivity type, which is constantly low- doped,
- a buffer layer (3) of the first conductivity type, which is arranged on the drift layer (2) towards the second main side (16) and which has a higher doping concentration than the drift layer (2), wherein the buffer layer (3) comprises towards the second main side (16) a high-doped region (36), which is constantly high-doped, and between the high- doped region (36) and the drift layer (2) an inter-space region (32), which is a diffused region and which has a doping concentration, which decreases steadily from the doping concentration of the high-doped region (36) to the low doping concentration of the drift layer (2), and
- a layer of the second conductivity type in form of a base layer (4) on the first main side (14).
9. Semiconductor device according to claim 8, characterized in, that the doping concentration of the inter-space region decreases by a Gaussian function.
PCT/EP2012/056456 2011-04-06 2012-04-10 Bipolar punch-through semiconductor device and method for manufacturing such a semiconductor device WO2012136848A1 (en)

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